It depends upon your cellular provider. Set up call forwarding or call waiting (GSM) provides an explanation:
- "Go to Settings > Phone.
- Tap any of the following:
- Call Forwarding: appears in the status bar when call forwarding is on. You must be in range of the cellular network when you set iPhone to forward calls, or calls won’t be forwarded.
- Call Waiting: If you’re on a call and call waiting is turned off, incoming calls go directly to voicemail.
- On models with Dual SIM, call waiting works only for incoming calls on the same line, unless the other line has Wi-Fi calling enabled and a data connection is available. See Set up cellular service on iPhone.
For CDMA accounts, contact your carrier for information about enabling and using these features.
For information about conditional call forwarding (if available from your carrier) when the line is busy or not in service, contact your carrier for setup information."